1. Field of the Invention
The present invention relates generally to the field of sensors, and more particularly relates to sensors for detecting analytes in fluids.
2. Description of the Prior Art
Sensors are widely used in technologies of detecting the presence of analytes in fluids. The following references are pertinent to this field of art:                1. U.S. Pat. No. 4,887,455 issued on Dec. 19, 1989 to Payne et al. for “Gas Sensor” (hereafter “the Payne '455 patent”);        2. U.S. Pat. No. 5,571,401 issued on Nov. 5, 1996 to Lewis et al. for “Sensor Arrays for Detecting Analytes in Fluids” (hereafter “the '401 Lewis patent”);        3. U.S. Pat. No. 6,319,724 issued on Nov. 20, 2001 to Lewis et al. for “Trace Level Detection of Analytes Using Artificial Olfactometry” (hereafter “the '724 Lewis patent”); and        4. Payne, et al., “High-Frequency Measurements of Conducting Polymers: Development of A New Technique for Sensing Volatile Chemicals”, Meas. Sci. Technol. 6 (1995) pp. 1500-1507 (hereafter “the Payne Publication”); and        5. U.S. Pat. No. 7,465,425 issued on Dec. 16, 2008 to Sun for “Sensor And Method For Detecting Analytes In Fluids” (hereafter “the Sun '425 patent”).        
The Payne '455 patent discloses a gas sensor that has a semiconductor organic polymer layer exposed to a gas to be detected. An analyzer applies an alternating electric signal at specific resonant frequencies to the sensor to detect the change in the sensor's impedance characteristics which is compared by a microcomputer with reference characteristics stored in a memory of the microcomputer. The gas in contact with the sensor can be detected because of the resulting difference spectra. The patent further discloses that the best performance of the invention is likely to be conducted between frequency ranging 100 MHz to 500 MHz wherein the resonance may happen.
The '401 Lewis patent discloses arrays of chemical sensors for detecting analytes in fluids. The sensors include first and second conductive elements electrically coupled to and separated by a chemically sensitive resistor which provides an electrical path between the conductive elements. The resistor includes a plurality of alternating nonconductive regions made of a nonconductive organic polymer and conductive regions made of a conductive material transverse to the electrical path. The resistor further provides a difference in resistance between the conductive elements when contacted with a fluid containing a chemical analyte at a first concentration, and then at a second different concentration. Arrays of such sensors are constructed with at least two sensors having different chemically sensitive resistors providing dissimilar such differences in resistance. Variability in chemical sensitivity from sensor to sensor is provided by qualitatively or quantitatively varying the composition of the conductive and/or nonconductive regions. An “electronic nose” for detecting an analyte in a fluid may be constructed by using such arrays in conjunction with an electrical measuring device electrically connected to the conductive elements of each sensor.
The '724 Lewis patent discloses a method using artificial olfactometry for detecting the presence of an analyte indicative of various medical conditions, including halitosis, periodontal disease and other diseases.
The Payne Publication discloses the change in the alternating current (AC) impedance characteristics of poly-N-(2-pyridyl) pyrrole in the presence of different volatile chemicals.
It can be seen from the above cited references that significant efforts have been devoted in the past in the research and development of sensors that are capable for detecting and identifying analytes in fluids. Identification of analytes in fluids from instrumental analysis is accomplished from mimic mechanisms of the mammalian olfactory system that applies probabilistic repertoires of many different receptors to record a single odorant.
However, identification of the odorant is dependent upon not only the results from highly specific receptors but also the output from less specific ones. In other words, identification is based on recognition a spectrum of signals that resemble a specific pattern. Following this direction, conventional technologies in sensor configuration were developed according to the following two schemes to generate a signal spectrum: applying multiple sensor and single sensor strategies.
In the approaches that utilize multiple sensors, various detecting devices have been developed that use metal oxide thin film resistor sensors, conductive polymer or polymer carbon powder composite film chemi-resistor sensors, polymer coated quartz crystal microbalance (QCM) sensors, polymer coated surface acoustic wave (SAW) sensors, metal-oxide-silicon field-effect-transistor (MOSFET) sensors, and optical sensors. However, although much progress has been made in the past, there are still primary disadvantages inherited from the sensing mechanisms of such multi-sensor technologies. The disadvantages include the requirement of a large number of sensors to generate a patterned information, the sophistication required for the sensor configuration, the poor reproducibility in sensor manufacturing, the strong humidity influence on chemical analysis, the slow response, the expensive electronics equipment required, and the very restricted operating conditions.
Various polymer films with a general thickness of several micrometers have been extensively used in multi-sensor configurations to improve sensor sensitivity and detection limit. This is primarily due to the fact that the polymer films can trap the chemical vapors because of their specific chemical selectivities on analytes. As a result, the analytes will be concentrated inside of the polymer prior to detection.
However, the conventional polymer films also inherit a number of disadvantages. First, the thin films of polymer are sensitive to the humidity associated with the analyte. Humidity is the predominant factor to influence performance of the polymer film based gas sensors. Second, polymer films have an aging effect that affects the sensor stability for long term operations. Third, it is difficult to achieve reproducibility of dispensing the polymer films onto sensors, particularly when a large number of sensors must be used in a multi-sensor configuration.
In the approaches that utilize a single sensor strategy, various instruments have been developed that are based on the mechanisms of gas chromatography (GC), mass spectrometry (MS), and light spectrum. Generally, these instruments are very expensive. Moreover, they are typically very bulky in sizes that makes miniaturization almost impossible. As a result, they are less attractive in the market where portability of instrument becomes increasingly important.
As an example, the Payne Patent and Publication discussed above disclose application of a single sensor for detecting impedance and phase sensitive components of conductive polymer modified electrodes at various frequencies to establish a spectrum of signals. However, the Payne device requires high frequencies ranging from 100 MHz to 500 MHz, which brings significant difficulties in instrument manufacturing and application. In addition, it still has the disadvantages inherent from polymer films.
In order to overcome deficiencies of the Payne technology and invent a new single sensor applying frequency sweeping, the '425 Sun patent discloses a single sensor as an analytical sensor for detecting analytes in fluids. The sensor is constructed from applying a pair of electrodes, wherein between the electrodes there are no additional materials designated to adsorb analytes, if their concentrations are high, or there are adsorbents, if the analyte concentrations are low. An alternating current voltage of varying frequencies is applied to the sensor by an alternating current device. In return, it detects electrical properties such as impedance and its components, reactance, resistance, and phase angles of the sensor with the analytes in fluids when they reside in or pass through the electrodes at each frequency. Thus two spectra of electrical properties of the analyte can be established at various applied frequencies from a single measurement. The electrical properties are analyzed by a pattern recognition process, and compared with those of the known Objects. Therefore, the analyte can be detected and identified. A reference sensor is provided with the same configuration of the analytical sensor. By combining electrical properties from the analytical and reference sensors, the '425 Sun patent provides a number of advantages, including elimination of humidity influence, polymer film aging effect, and electrical property variations caused by the temperature variations.
Therefore, it is desirable to design and develop a new sensor and method that overcome the disadvantages of conventional sensor devices, and have a better reproducibility of performance and sensor manufacturing, fewer interference deficiency, enhanced sensitivity, less restricted operation conditions, and increased portability.